Since the beginning of research into the field of combustion technology, great importance has been placed on the determination of flame temperatures. The flame temperature is a key parameter in the combustion of fossil fuels, since it correlates directly with the chemical reaction kinetics and the formation of pollutants, for example NO.sub.x. Moreover, knowledge of the release of energy during the combustion process is indispensable for designing combustion chambers and determining mechanical and thermal stresses on all the components that are involved.
There are currently a multiplicity of techniques for measuring flame temperatures. In this context, the extreme working conditions are, in particular, very challenging to temperature sensors, and so it does not directly follow that any temperature sensor tested under neat laboratory conditions can be employed in an industrial combustion chamber.
Contemporary temperature measurement techniques can be broadly divided into two categories, one of which uses non-optical temperature sensors and the other optical ones.
Included among non-optical temperature measurement devices are point sensors which, for example, comprise thermocouples. They afford a simple and economical possibility of determining temperature at discrete points, but need to be installed in direct proximity to the flame, and therefore affect the flame. In addition, because of their fragility, thermocouples are of only limited use in a turbulent high-temperature environment, in which chemical surface reactions additionally damage the thermocouples.
In particular since laser technology has become known, a large number of optical temperature measurement devices with correspondingly tailored measurement methods have been developed. Amongst others, these include absorption and fluorescence methods, as well as various measurement methods which make use of the scattered laser light. A common factor with the abovementioned optical measurement methods is that they need a light source, namely a laser. They are therefore active in nature, but in contrast to thermocouples, they have no effect on the flame. These methods infer the temperature of a flame while taking into account the light emitted by the source and the measurement volume. However, this technique is expensive in view of the measuring devices that are used and the costs resulting therefrom. Commercial use of such laser-assisted measuring systems is therefore very limited.
A known non-active optical flame temperature determination is carried out by means of pyrometry, use being made of black-body radiation emitted by soot particles contained in the flame. However, it is difficult to use pyrometric temperature measurement systems for flames of gaseous fuels. The optical system is in this case very weak because of the low soot content. During the signal analysis, a further difficulty arises since the temperature- and wavelength-dependent emissivities of the radiating soot particles is only approximately known, which, in combination with undesired absorption effects on the path leading to the detector, impair the accuracy of the method.
JP-A-60 036 825 discloses a temperature measurement method for determining the temperature of a flame. In this method, the vibration spectrum of the OH radicals is measured in order to determine the temperature. For this, the emission intensity of discrete spectral lines of the OH radical transition is determined by spectral analysis.
The article "Rotational Temperature Estimation of CO at High Temperatures by Graphical Methodes Using FT-IR-Spectrometry", of Mc Nesby et al., published in "Applied Spectrometry", vol. 45, no. 1, 1. January 1991, discloses a method for determining the temperature by determining the rotational spectra of CO in a flame. For this, the line spectra of a single R-Branch is determined, which is submitted to a correction.
In summary, it can be stated that currently available flame temperature measurement methods are of only limited industrial use, since either they are cost-intensive, or they do not ensure sufficient stability at the required accuracy, in view of contamination.